Semiconductor integrated circuits, such as processors, microcontrollers, memories, etc., incorporate fuse elements that allow individual configuration of each chip. Examples of parameters capable of being adjusted on a per chip basis are internal timings, supply voltage level, chip identification number, repair information, and the like. In particular, the latter may be used in memory circuits (e.g., DRAMs) to control the manner in which spare elements are used to replace defective elements. With the ever-growing memory capacity of these chips, the number of fuses has also increased. For example, a typical 64 MB DRAM circuit may contain a few thousand fuse elements.
Commonly, a fuse is evaluated by a fuse latch circuit, which translates the analog resistance value of the fuse link into a digital value (“high” or “low”). In addition, the fuse latch stores that digital value. In a typical implementation of a fuse latch, the storing functionality is implemented by using a pair of cross-coupled inverters. The input signal of the first inverter is connected to a precharge switch, which is controlled by a precharge signal, and to a read switch, which is controlled by a read signal. The read switch connects the fuse latch input to one terminal of the fuse element, and the other terminal of the fuse element is connected to the ground power supply. The output of the first inverter provides the fuse latch output, and is also connected to the input of the second inverter. The second inverter output is connected back to the input of the first inverter, thus providing a latch function.
Generally, a fuse evaluation procedure consists of two steps, a precharge operation and a read operation. The precharge operation initializes the fuse latch by precharging the input of the fuse latch prior to the reading of the actual fuse value. Then the read operation turns on the read switch to connect the fuse latch to the fuse terminal. A low fuse resistance will cause the fuse latch to switch its state, whereas a high fuse resistance will not cause a change of state. In effect, the fuse element, the read switch, and the second inverter are resistive elements that form a resistive divider network between the power supply and ground. The latch circuit responds to the voltage at its input, which is a function of the resistance of the resistive elements in the voltage divider.
At the end of a fuse evaluation procedure, the output of the fuse latch is indicative of the state of the fuse. Generally, for example with laser or electrical fuses, a low resistance indicates an unblown fuse, whereas a high resistance indicates a blown fuse. Fuse blowing is typically performed by applying energy (e.g., laser beam, electrical current) to the fuse link. As a result, the fuse link is intentionally damaged and exhibits a higher resistance compared to its resistance before the fuse blow. Commonly, the characteristics of the latch are summarized by a single value, the trip point of the latch. If the fuse resistance is below the latch trip point, the latch output will be high to indicate an unblown fuse. On the other hand, if the fuse resistance is above the latch trip point, the latch output will be low to indicate a blown fuse.
Ideally, the fuse latch trip point is above the unblown fuse resistance and below the blown fuse resistance. These three resistance values (fuse latch trip point, unblown fuse resistance, and blown fuse resistance) are not constant, but are affected by variations in other parameters. These parameters may include, for example, the operating temperature, the manufacturing process of the fuse latch, and the power supply voltage. For reliable operation of the fuse latch, it is desirable that the three resistance values do not overlap and that a sufficient margin between the values exists. Otherwise, if an overlapping situation is present, a blown fuse might be incorrectly indicated as unblown by the fuse latch, or an unblown fuse might be incorrectly indicated as blown by the fuse latch.
In the prior art, however, generally the only accessible value in a fuse latch is the fuse latch output signal. This signal is a digital value that is either in a high or a low state, and therefore does not provide information on the margin between the resistance of an unblown fuse and the trip point, nor does it provide information on the margin between the trip point and the resistance of a blown fuse. Therefore, a prior art fuse latch circuit generally cannot be properly tested for reliability of operation.